A hot-rolled steel strip is covered with mill-scales mainly composed of oxides. If the hot-rolled steel strip is subjected as it is to further processing steps such as cold-rolling, it leads to the occurrence of defects such as surface flaws and cracks caused by the mill-scales. In this consequence, the scales are generally removed from the surface of the hot-rolled steel strip by pickling, before the hot-rolled steel strip is subjected to further processing steps. In this process, there are problems on a pickling section, recycling process of waste acids, adjustment of descaling capability etc. There is also the fear that the properties of the steel material will deteriorate due to the penetration of hydrogen produced during pickling.
In order to solve these problems, there have been studied various methods of removing scales from the surface of a hot-rolled steel strip, before the hot-rolled steel strip is subjected to pickling. For instance, the step of cold-rolling a hot-rolled steel strip at a heavy rolling reduction (hereunder referred to as "mill-scale rolling") is disclosed in Japanese Patent Publication 54-133460, Japanese Patent Application Laid-Open 57-41821 and Japanese Patent Application Laid-Open 57-10917. Cracks are formed in the scales by the mill-scale rolling, and the adhesiveness of the scales to the steel strip is weakened, so as to facilitate the removal of the scales from the surface of the cold-rolled steel strip by shot blasting, high-pressure water spraying, brushing, grinding with abrasive grains, etc. As a result, the amount of scales adhering to the hot-rolled steel strip to be carried to the pickling tank is reduced, with a consequent reduction in the load on the pickling step.
Although the load on the pickling step is certainly reduced when the hot-rolled steel strip is subjected to the mill-scale rolling, there is the tendency for scale fragments which separated from the surface of the steel strip to become adhered to the surface of rolls, such as the bridle rolls, in latter steps, and subsequently become re-adhered onto the surface of the steel strip. The scales in this case are different from the scales present on the surface of the steel strip which is passed through a tension leveller, in that their adhesiveness to the surface of the steel strip is strong. Consequently, the amount of scales carried into the pickling tank is large, so as not to realize a reduction in the load on the pickling step as large as anticipated.
Furthermore, scale fragments which had once separated from the surface of the hot-rolled steel strip by the mill-scale rolling but then become firmly re-adhered or pressed back onto the surface of the steel strip, are difficult to remove in the pickling step and often tend to cause defects such as surface flaws in a subsequent cold-rolling step. Although grinding with abrasive grains has been used to try to remove the scale fragments, there are always some left remaining on the surface of the steel strip.
The inventors have carried out various studies into countermeasures to remove these residual scales which cause surface flaws in the product with the aim of exploiting the advantages of the mill-scale rolling which is effective in reducing the load on the pickling step. As a result thereof, the inventors found that when a hot-rolled steel strip is cold-rolled at a large rolling reduction under specified conditions, mill-scales can be efficiently eliminated from the surface of the steel strip with a resulting remarkable reduction in the load on the subsequent pickling step.
The present invention has been completed on the basis of the results of our investigation and research into the effects of heavy-duty cold-rolling on the peelability of mill-scales. The object of the present invention is to reduce the amount of mill-scales fed to a pickling tank, and to thereby deliver to subsequent steps a steel strip whose load on the pickling step has been reduced.